Professor G. S. West has studied the reproduction of this species and finds that it is a true Zygnema and that the description and figure by De Bary, which shows the cutting off of two special gametangia before the union of the gametes is at fault, consequently there is no longer any need of maintaining the genus Zygogonium Kützing.
Z. peliosporum Wittrock.
Specimens of this species have been distributed under the name of Z. chalybeospernum Hansgirg, in P. B.-A. No. 808 from Boswell, Calif. (N. L. Gardner); Amer. Alg. No. 156 from Ft. Collins, Colo. (J. H. Cowan); and Amer. Alg. No. 392 from Vancouver, B. C. (J. E. Tilden). Z. chalybeospermum has the median wall smooth, but the spores of all of the above specimens have distinctly scrobiculate median walls. In size the specimens show a somewhat greater variation in dimensions than has been recorded for European localities.
Z. cruciatum (Vauch) Agardh.
Specimens of this species have been found at Fath Pond, north of Coffeen, Ill., in which both zygospores and aplanospores occurred in abundance. The aplanospores fill or slightly enlarge the vegetative cells as in Z. Collinsianum Transeau,[C] but the ends of the spores are usually more nearly truncate, 34-50µ × 30-80µ. At Casey, Ill., a variety with the same dimensions but steel blue spores occurs in the old Ice Plant Pond.
var. caeruleum nov. var. Cellulis vegetativis et sporis ut in typo, sed membrana media sporarum caerulea.
Vegetative cells and spores as in the type, except that the median spore wall is steel blue. Type in Collection E. N. T. No. 495.
Zygnema stellinum (Müller) Agardh.
The specimen distributed under the name Z. insigne (Hass.) Kütz. in the P. B.-A. No. 457, from Chestnut Hill, Mass. (G. F. Moore), belongs to this species as shown by the scattered mature spores. This species is common everywhere in central Illinois. In the U. S. Natl. Herb, is a specimen from Baltimore Co., Md., (J. D. Smith). In Amer. Alg. No. 157, a specimen from St. Paul, Minn., (J. E. Tilden) shows both zygospores and aplanospores. The aplanospores are cylindric ovoid in form, occupying the entire cell 30-33µ × 40-88µ, median wall scrobiculate.